Adeno associated Virus (AAV) is a small and helper dependent virus. It was discovered in 1960s as a contaminant in adenovirus (a cold causing virus) preparations. Its growth in cells is dependent on the presence of adenovirus and, therefore, it was named as adeno-associated virus. Before 2002, a total of 6 serotypes of AAVs were identified, including the serotype 2 which was the first AAV developed as vector for gene transfer applications and the one used in the recent break through eye gene therapy trials. In the earlier attempts to develop AAV as gene transfer vehicle, prototype AAV vector based on serotype 2 effectively served as a proof-of-concept showcase and accomplished non-toxic and stable gene transfer in murine and large animal models in different target tissues. For instance an 8 year stable vision improvement was observed in a dog model of LCA after a single injection and a 9 year stable gene expression in Macaque muscle was achieved. However, these proof-of-concept studies also revealed a significant shortcoming which is a poor gene transfer efficiency in major target tissues.
Methods for discovering novel AAVs have been largely focused on isolating DNA sequences for AAV capsids, which relate to the tissue targeting capacity of the virus. To date, the principal methods employed for identifying novel AAV take advantage of the latency of AAV proviral DNA genomes and focus on rescuing persisted viral genomic DNA. The major challenge in DNA-targeted AAV isolation is that the abundance of persisted AAV genomes is often very low in most of tissues particularly in human tissues, which makes AAV rescue unachievable in many cases.